A non-recovery coke oven is used to convert coal into blast furnace grade coke by heating the coal in a reducing atmosphere and operating under negative pressure. The oven is heated by external means to 2500.degree. F. The external heat is shut off and a bed of coal is charged to the floor of the oven. The surface of the coal bed immediately generates combustible gas, herein referred to as "coal gas," by the radiant energy absorbed by the oven roof from the external heat source. Approximately one-third (1/3) of the gas given off by the coal is selectively burned in a "primary" combustion by allowing "primary air" to be drawn into the oven interior through dampers in the doors of the oven. Next the combustion products and the remaining two-thirds (2/3) of the combustible gas are drawn through downcomers in the side walls of the oven into the sole flues, which are cavities below the oven floor for the combustion of gas. "Secondary air" is drawn into the sole flues to burn the remaining gases in a "secondary" combustion.
The heat generated by the primary combustion in the oven interior and the secondary combustion in the sole flues, provides the total heat to convert the coal into coke. Two independent thermal gradients occur, one beginning at the top of the coal bed and progressing downward and one beginning at the oven floor progressing upward. The proportion of primary and secondary air also controls the rate at which the thermal energy proceeds through the coal bed.
In the production of coke it is desirable to have a predominant secondary or "sole flue" coke thermal gradient beginning at the floor and progressing upward. The coke produced under this condition tends to be stronger and of high quality. The coke at the top of the bed resulting from the primary thermal gradient (called "bee-hive" coke) contains more air pockets and is somewhat spongy.
Controlling the amount of primary air in the oven interior is important. This is accomplished by manipulating air dampers in the oven doors. However, the volume of air that passes through a damper is not only a function of damper position, but also the negative pressure behind the damper which draws the air into the interior cavity of the oven. Oven interior pressure can be so different from outside pressure that even a damper position creating a small opening can overwhelm the cavity with primary air.
To solve the problem of pressure control, a valve in the downcomer passage or a "downcomer valve" acts as a variable restrictor which increases the pressure drop across the face of the downcomer. This permits a precise reduction of pressure in the interior of the oven which allows greater control of the inlet of primary combustion air.